A Shepard-class Federation starship under Captain Kovil, destroyed by cloaked Klingon vessels in 2256 despite the USS Discovery's attempt to defend it.
The USS Gagarin was a Shepard-class starship named after the cosmonaut Yuri Gagarin. Its captain during the war of 2256 was Kovil.
Its loss opens the Discovery episode Si Vis Pacem, Para Bellum. The Gagarin is ambushed by a group of Klingon ships, and Discovery jumps in to help, putting itself between the attackers and the damaged ship. The Klingons, who by now all carry the cloaking device first used by T'Kuvma's flagship, appear and disappear at will. One of them gets a clear shot and the Gagarin is destroyed with all hands.
The defeat shows how badly the war is going now that every Klingon house has the cloak, and it sends Burnham, Saru and Tyler to Pahvo to turn the planet's natural transmitter into a means of detecting cloaked ships.